The following abbreviations and terms are herewith defined:
3GPP 3rd generation partnership project
AP access point
BS base station (e.g., network access node, NodeB, eNodeB, etc.)
D2D device-to-device
IMT international mobile telecommunications
ITU international telecommunications union
LTE long term evolution of UTRAN (also known as 3.9G)
OFDM orthogonal frequency division multiplex
SIR signal-to-interference ratio
UE user equipment (e.g., mobile or subscriber station SS/MS, terminal)
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
WiMAX worldwide interoperability for microwave access
Currently the IMT-Advanced process tries to guide the development of future cellular wireless access in order to fit future user needs. IMT-Advanced include radio technologies that meet the requirements currently defined by ITU for radio technologies beyond IMT-2000 (year 2010 and beyond). 3GPP is currently defining a study item to prepare LTE-Advanced that meets the IMT-Advanced requirements. Competing technologies such as WiMAX are expected to define advanced versions of current standards to be IMT-Advanced technologies. For WiMAX, standardization of IMT-Advanced technology is currently taking place in the 802.16m task group.
Device-to-device D2D communication has been identified as one of the possible areas of study for future cellular enhancements leading to IMT-Advanced, in order to enable new type of services. D2D communication has been mentioned and discussed during the IMT-A workshop organized by 3GPP in Shenzhen, China, in April 2008, and suggested by Motorola to the 802.16m task group.
There are several standards that support D2D operation in the same broad frequency bandwidth as is used by the network access point, base station or central controller as the case may be. Some of these are detailed below with reference to FIG. 1a in which two portable wireless devices UE1 and UE2 are communicating with the network base station BS and with one another.
In the Hiperlan 2 (high performance radio local area network, similar to IEEE 802.11 system), UE1 sends a resource request (several OFDM symbols=slots) for direct communication with UE2 to the central controller/BS. After receiving a resource grant, UE1 transmits to UE2 in the granted slots within the direct link phase in the MAC (medium access control) frame. If UE2 wants to transmit to UE1 it has to reserve slots as well. An exception exists in acknowledged mode, where the central controller reserves also slots for the acknowledgements of the other UE, but still the central controller reserves the slots used in communications between UE1 and UE2. It is also possible for a UE to request a fixed slot allocation, i.e. selected slots are allocated to a UE for multiple frames instead of on an individual slot basis. Note that in Hiperlan 2, the allocation is always for a single UE and the central controller as well as any other UEs communicating directly with one another cannot transmit at the same time. This is not seen to be an efficient use of the available radio resources. Each UE has to reserve slots for each and every transmission which results in a high signaling load for requesting and allocating the slots. The number of direct links in the subnet is limited in the fixed slot allocation. Further, only full OFDM symbols can be reserved, which is too much for a system bandwidth of e.g. 100 MHz with 2048 subcarriers. For example, assuming 1600 usable subcarriers and 64QAM modulation, this equals to 8 kb for one OFDM symbol but for example a TCP/IP acknowledgement packet has only a size of 320b. So in Hiperlan 2, if one OFDM symbol is reserved for direct communications between UE1 and UE2, then no other UE in the same subnet (under the same BS) is able to communicate using the same OFDM symbol. While this restriction ensures that there is no interference from another node in the subnet, it is not seen as the best use of scarce radio resources.
In the Tetra system (terrestrial trunked radio, designed for use by government agencies and emergency services), several frequency channels are reserved purely for device to device communication. However this system uses a fixed allocation of channels for D2D communications, which reduces the amount of resources available for the BS-UE links.
In the WLAN system, UE1 senses the medium and if it is free, it transmits. But there is no control by the network over the D2D links and so as spectrum becomes more crowded there will be insufficient free medium for D2D communications.
In the WiMAX system, there is a recent proposal of Motorola in the 802.16m study group to reserve a zone (several full OFDM symbols) for D2D communication. Similar to Hiperlan 2, only full OFDM symbols can be reserved which as noted above and by example is too much for a system bandwidth of e.g. 100 MHz with 2048 subcarriers.
As can be seen from the above review, a common assumption in those existing systems is that the D2D communications occur in frequency and/or time resources that are separated from those in use by the network (BS) directly. However IMT-A anticipates that D2D communication will share the same band that the cellular network is using, and hence it is needed to coordinate the D2D communication with the cellular network to be able to offer guaranteed service levels to the users in the cellular network. What is needed is a way to enable D2D communications in a manner that does not unnecessarily limit radio resource available to the network and that allows the network to fulfill its minimum requirements as to channel quality (quality of service QoS; error rate, etc.).